Performance Investigation of a PV/T
Component Used as a Part of the Building Envelope
Herman Andersen Joseph Ekenes
Master of Energy Use and Energy Planning Supervisor: Vojislav Novakovic, EPT
Co-supervisor: Yanjun Dai, Shanghai Jiao Tong University Laurent Georges, EPT
Department of Energy and Process Engineering Submission date: July 2017
Norwegian University of Science and Technology
v
Preface
This master’s thesis was conducted as a part of the study program Energy and Environmental Engineering at the Norwegian University of Science and Technology (NTNU). The thesis is a part of the Joint Research Center in Sustainable Energy of NTNU Shanghai Jiao Tong
University (SJTU). The work was carried out during the spring semester of 2017 in Shanghai.
The main objective is to analyse and develop design methods for the novel solar PV/T component installed as a part of the building envelope at the Green Energy Laboratory at SJTU.
The authors would like to thank our supervisor, Professor Vojislav Novakovic for the opportunity to be part of the collaborative research and for consultations throughout the semester. Also, we would like to thank our co-supervisor Professor Yanjun Dai for guidance during our stay at SJTU. Moreover, we would like to thank Professor Laurent Georges for technical support regarding the use of TRNSYS.
Joseph Ekenes Shanghai 2017
Herman Andersen
vi
Abstract
The goal of this master thesis is to analyse and develop design methods for building
integrated photovoltaic/thermal (BIPV/T) technology at the Green Energy Laboratory (GEL) at Shanghai Jiao Tong University (SJTU) in China.
PV/T technology generates electrical and thermal energy in a smaller area, compared to solely PV panels and solar collectors. For a PV panel, the electrical efficiency will decrease when the surface temperature increases, typically during peak solar irradiation. The air or water circulating in the PV/T component cools the PV surface, maintaining higher efficiency and thus higher energy generation.
A water based PV/T component has been calibrated and validated according to measurements conducted in Shanghai, China. Furthermore, the component was used to model a PV/T façade at the south wall of GEL. A façade integrated PV/T system utilising air as heat transfer medium was also modelled, but not validated, as no measurements were available for this component.
The BIPV/T systems were optimised for five parameters; dead band, storage tank size, mass flow rate, tank inlet height from heat source and tank inlet height from mains water supply.
Simulations have been conducted to analyse the effect of building integration, both on the building energy demand and the BIPV/T system operation. Simulations were carried out for the same building model with air based BIPV/T system, water based BIPV/T system, air based PV/T system, water based PV/T system and PV façade (BIPV).
The results show that PV/T integrated to the building façade has negligible effect on the total energy demand of the building. The electrical efficiency was highest for the air based BIPV/T system and the water based BIPV/T showed the largest amount of collected thermal useful energy. The BIPV showed the highest electrical solar fraction, as a significant amount of fan energy required for operation of the air based BIPV/T system results in reduced solar fraction for that system.
vii
Sammendrag
Målet med denne masteroppgaven er å analysere og undersøke metoder for design av bygningsintegrert PV/T (BIPV/T) teknologi ved Green Energy Laboratory (GEL) ved Shanghai Jiao Tong University (SJTU) i Kina.
PV/T teknologi kan generere både elektrisk og termisk energi på et mindre areal,
sammenlignet med kun PV-paneler eller solfangere. For et PV-panel vil virkningsgraden synke med økende overflate temperatur, som typisk inntreffer under maksimal solinnstråling.
Vann eller luft som sirkuleres inne i PV/T-komponenten vil kjøle ned PV-overflaten, og på den måten opprettholde høyere virkningsgrad og høyere energiproduksjon.
En vannbasert PV/T-komponent er kalibrert og validert i henhold til målinger utført i Shanghai, Kina. Videre har komponenten blitt brukt til å modellere en PV/T-fasade på sørveggen av GEL. Et fasadeintegrert PV/T-system med luft som kjølemedium er også modellert, men ikke validert, da måledata for denne komponenten ikke var tilgjengelig.
BIPV/T-systemene er optimalisert for fem parametere; dødbånd, størrelse på varmtvannstank, massestrøm, tankinnløp varmekilde og tankinnløp byvann. Simuleringer er utført for å
analysere effekten av bygningsintegrering, både på byggets energibehov og på BIPV/T systemet. Like simuleringer er gjort for samme bygningsmodell med luftbasert BIPV/T- system, vannbasert BIPV/T-system, luftbasert PV/T-system, vannbasert PV/T-system og PV- fasade (BIPV).
Resultatene viser at et fasadeintegrert PV/T-system har neglisjerbar innvirkning på byggets totale energibehov. Elektrisk virkningsgrad var høyest for det luftbaserte BIPV/T-systemet, mens mengden generert termiske energien var høyest for det vannbaserte systemet. PV- fasaden oppnådde høyest andel generert solenergi i forhold til energiforbruk for systemet, ettersom energiforbruket til viften førte til lavere andel i det luftbaserte BIPV/T-systemet.
viii
1 Table of Contents
Abstract ... vi
Sammendrag... vii
List of Figures ... xii
List of Tables ... xvi
1 Introduction ... 1
1.1 Objective ... 1
1.2 Background ... 1
1.3 Limitations ... 2
1.4 Outline ... 3
1.5 Research Methods ... 4
2 Theoretical Background ... 5
2.1 Photovoltaic ... 5
2.1.1 Manufacturing ... 6
2.1.2 Photovoltaic Effect ... 7
2.1.3 Monocrystalline ... 8
2.1.4 Multicrystalline ... 8
2.1.5 Thin Film ... 8
2.1.6 Performance of Photovoltaics ... 9
2.2 Solar Thermal Collectors ... 12
2.2.1 Flat Plate Solar Collector ... 13
2.2.2 Performance of Solar Collectors ... 14
2.3 Building Integrated PV/T Component ... 16
2.3.1 Air Based BIPV/T ... 17
2.3.2 Water Based BIPV/T ... 19
2.3.3 Performance of BIPV/T ... 19
2.3.4 Sky temperature ... 21
2.3.5 Cover heat loss ... 21
ix
2.4 Net Zero Energy Buildings ... 21
2.4.1 Building Design ... 25
2.4.2 Load Matching ... 25
2.4.3 Smart Grids ... 27
2.5 BIPV/T System Design ... 27
2.5.1 BIPV/T Component Tilt Angle ... 27
2.5.2 BIPV/T Component Area ... 28
2.5.3 Thermal Energy Storage ... 29
2.5.4 Electrical Energy Storage ... 31
2.5.5 Mass Flow Rate ... 32
2.5.6 BIPV/T Components Connected in Parallel and Series ... 33
2.5.7 DC/AC Converter (Inverter) ... 35
2.5.8 Heat Pump Coupled BIPV/T Component ... 35
3 The Green Energy Laboratory ... 39
3.1 GEL Façade ... 40
3.2 GEL Energy Systems ... 41
3.2.1 Open-loop Surface Water Source Heat Pump System: ... 42
3.2.2 Ground-coupled Heat Pump System: ... 42
3.2.3 CO2 Heat Pump: ... 43
3.3 GEL Office ... 43
3.4 Measurements ... 44
4 Modelling and Simulation Tools ... 47
4.1 Mathematical Model of PV/T ... 49
4.1.1 PV/T Water Model ... 51
4.1.2 PV/T Air Model ... 57
4.1.3 Radiative Heat Transfer ... 64
4.1.4 Convective Heat Transfer ... 66
4.1.5 Weather File ... 68
x
4.1.6 Pump ... 69
4.1.7 Storage Tank ... 70
4.2 Validation of the TRNSYS PV/T Component ... 70
4.3 Calibration of the TRNSYS PV/T Component ... 72
4.3.1 Baseline Model ... 72
4.3.2 Calibration Measures ... 76
4.3.3 Calibrated Model ... 79
4.4 Performance Evaluation of Calibrated Model ... 87
4.4.1 Thermal Efficiency ... 87
4.4.2 Electrical Efficiency ... 90
4.4.3 Total Efficiency and Exergy Calculations ... 90
4.5 GEL Model ... 92
5 Energy Simulations of the GEL Model ... 94
5.1 GEL Office ... 96
5.2 Multizone Model (TYPE 56) ... 98
5.2.1 Building Envelope ... 100
5.2.2 Modelling of the BIPV/T Wall ... 100
5.2.3 Schedule ... 101
5.2.4 DHW load ... 101
5.2.5 Internal Gains and Electrical Load ... 101
5.2.6 Ventilation ... 102
5.2.7 Heating and cooling ... 102
5.2.8 Differential controller (TYPE 2) ... 103
5.2.9 Storage Tank (TYPE 60) ... 104
5.3 Optimisation of GEL Models ... 105
5.4 Water Based BIPV/T System ... 105
5.4.1 Dead Band ... 107
xi
5.4.2 Size of Storage Tank ... 109
5.4.3 Specific Flow Rate ... 111
5.4.4 Inlet from Heat Source ... 113
5.4.5 Inlet from Cold Side ... 115
5.4.6 Summary ... 116
5.4.7 Optimised GEL model ... 119
5.5 Air Based BIPV/T System ... 119
5.5.1 Dead Band ... 121
5.5.2 Size of Storage Tank ... 123
5.5.3 Specific Flow Rate ... 124
5.5.4 Inlet from Heat Source ... 127
5.5.5 Inlet from Cold Side ... 128
5.5.6 Summary ... 129
5.5.7 Optimised GEL model ... 132
6 Performance Analysis of GEL BIPV/T System Models ... 133
6.1 Thermal Performance of BIPV/T ... 133
6.2 Summary ... 136
6.3 Electrical Performance of BIPV/T ... 136
6.4 Mismatch factors ... 139
6.5 Summary ... 142
6.6 Effect of Building Integration ... 143
6.7 Summary ... 144
7 Conclusion ... 145
8 Further Work ... 147
References ... 148
Appendix A: Energy Balances ... 152
Appendix B: GEL Office Parameters... 154
xii
List of Figures
Figure 2-1: Mean annual global irradiance incident on a horizontal surface in W/m2 [] ... 5
Figure 2-2: Photovoltaic cell [6] ... 7
Figure 2-3: Market shares of photovoltaic technologies. Data from [7] ... 9
Figure 2-4: Highest measured PC cell efficiencies [9] ... 10
Figure 2-5: Covered flat plate collector [13] ... 13
Figure 2-6: Market shares of installed solar collector capacity. Data from [16] ... 14
Figure 2-7: Overview of various BIPV/T technologies [17] ... 17
Figure 2-8: Air based BIPV/T component ... 18
Figure 2-9: Open loop (a) and closed loop (b) air based BIPV/T systems. Adapted from [13] ... 18
Figure 2-10: Water based BIPV/T component. Flow direction into the page ... 19
Figure 2-11: Graphical representation of the net-zero energy balance concept. Adapted from [35] .... 22
Figure 2-12: Exported/delivered and load/generation balances. Adapted from [35] ... 23
Figure 2-13: General monthly graphs of electric energy load and generation of a building. Adapted from [37] ... 26
Figure 2-14: Stratified tank with a BIPV/T component side heat exchanger ... 31
Figure 2-15: Electrical configuration of BIPV/T system [5] ... 34
Figure 2-16: Series and parallel connections of BIPV/T [5] ... 35
Figure 2-17: Ground source heat pump operation during cooling and heating season ... 36
Figure 2-18: Series configuration of a BIPV/T and a ground source heat pump system. Adapted from [54] ... 37
Figure 2-19: Parallel configuration of a BIPV/T and a ground source heat pump system. Adapted from [54] ... 38
Figure 3-1: The Green Energy Laboratory [57] ... 39
Figure 3-2: Atrium showed in picture (left) and on floor plan (right) [57] ... 40
Figure 3-3: The two layers of the facade (left) and the external facade configuration (right) [56] ... 40
Figure 3-4: GEL façade operation during summer and winter [58] ... 41
Figure 3-5: Energy technologies installed at the GEL. Adapted from [56]... 42
Figure 3-6: Floor plan of the 2nd floor in the GEL ... 43
Figure 3-7: Ambient conditions during the PV/T water measurements ... 45
Figure 3-8: Solar test rig with one PV/T component connected to a tank ... 46
Figure 4-1: Comparison of real measurements to various simulation tools for PV production [61] .... 48
Figure 4-2: Modelling process. Adapted from [62] ... 49
Figure 4-3: TRNSYS system sketch of the baseline model ... 51
Figure 4-4: Water based PV/T component ... 52
Figure 4-5: Cover energy balance of water based PV/T ... 52
xiii
Figure 4-6: Fin effect in the water based PV/T ... 54
Figure 4-7: Energy balance in the fin area of the absorber ... 55
Figure 4-8: Energy balance of the fin base of the absorber ... 55
Figure 4-9: System sketch showing the temperature nodes of the air based PV/T ... 58
Figure 4-10: Cover energy balance of air based PV/T ... 58
Figure 4-11: Energy balance at the upper surface of the air channel ... 60
Figure 4-12: Energy balance at the lower surface of the air channel ... 61
Figure 4-13: Differential balance of the air flow ... 62
Figure 4-14: Sky temperatures calculated by various methods ... 66
Figure 4-15: Instant wind velocity at the GEL. ... 67
Figure 4-16: Convective heat transfer coefficients as a function of wind speed by various authors .... 68
Figure 4-17: Stratified storage tank (TYPE 4) ... 70
Figure 4-18: Power output of the measured data and the baseline model ... 73
Figure 4-19: PV/T outlet temperatures of the measured data and the baseline model ... 74
Figure 4-20: Calibration signatures of the baseline model ... 75
Figure 4-21: Characteristic signatures of PV/T validation metrics when implementing sky temperature by the method of Berdahl and Martin... 77
Figure 4-22: Characteristic signature of the PV/T outlet temperatures when implementing hwind by the method of various authors. ... 78
Figure 4-23: Characteristic signatures of the power output when implementing hwind by the method of various authors. ... 78
Figure 4-24: Calibration signatures of the validation metrics for the calibrated model. ... 80
Figure 4-25: Outlet temperature of the calibrated and baseline models compared to the measured data ... 81
Figure 4-26: Power output of the calibrated and baseline models compared to the measured data ... 82
Figure 4-27: Residual distribution of the PV/T outlet temperature from the baseline model with a fitted normal distribution curve ... 84
Figure 4-28: Residual distribution of the PV/T outlet temperature from the calibrated model with a fitted normal distribution curve ... 85
Figure 4-29: Residual distribution of the PV/T power output from the baseline model with a fitted normal distribution curve ... 86
Figure 4-30: Residual distribution of the PV/T power output from the calibrated model with a fitted normal distribution curve ... 87
Figure 4-31: Thermal efficiencies of the calibrated model and the measured data ... 88
Figure 4-32: Thermal efficiencies of the calibrated model, the baseline model and the measured data during the hours of positive efficiency values ... 89
xiv
Figure 4-33: Electrical efficiencies of the calibrated model, the baseline model and the measured data
... 90
Figure 4-34: Total efficiencies of the baseline model, the calibrated model and the measured data .... 91
Figure 4-35: Total exergy efficiencies for the baseline model, the calibrated model and the measured data ... 92
Figure 5-1: TRNSYS model of the water based BIPV/T system ... 94
Figure 5-2: TRNSYS model of the air based BIPV/T system ... 95
Figure 5-3: Solar irradiance for a vertical surface facing south and a horizontal surface ... 96
Figure 5-4: Ambient temperature and mains water temperature ... 97
Figure 5-5: Illustration of a real wall and the corresponding black box model ... 99
Figure 5-6: DHW profile (left) and electrical load profile (right) as set in TRNSYS ... 102
Figure 5-7: Annual heating (negative values) and cooling (positive values) demand ... 103
Figure 5-8: Controller function ... 104
Figure 5-9: Monthly values of thermal and electrical efficiencies when varying the dead band ... 108
Figure 5-10: Monthly values of useful energy (column) and operational hours (line) when varying the dead band ... 109
Figure 5-11: Monthly values of thermal and electrical efficiency when varying the storage tank volume ... 110
Figure 5-12: Monthly values of useful energy (column) and operational hours (line) when varying storage volume ... 111
Figure 5-13: Monthly values of thermal and electrical efficiency when varying the specific flow rate ... 112
Figure 5-14: Monthly values of useful energy (column) and operational hours (line) when varying the specific flow rate ... 113
Figure 5-15: Monthly values of thermal and electrical efficiency when varying the inlet height from the heat source ... 114
Figure 5-16: Monthly values of useful energy (column) and operational hours (line) when varying the inlet height from the heat source ... 114
Figure 5-17: Monthly values of thermal and electrical efficiency when varying the inlet height from the cold side ... 115
Figure 5-18: Monthly values of useful energy (column) and operational hours (line) when varying the inlet height from cold side ... 116
Figure 5-19: Monthly values of thermal and electrical efficiency when varying dead bands ... 121
Figure 5-20: Monthly values of useful energy (column) and operational hours (line) when varying the dead band ... 122
Figure 5-21: Monthly values of thermal and electrical efficiency when varying the storage tank volume ... 123
xv
Figure 5-22: Monthly values of useful energy (column) and operational hours (line) when varying the
storage volume ... 124
Figure 5-23: Monthly values of thermal efficiency when varying the specific flow rate ... 125
Figure 5-24: Monthly values of electrical efficiency when varying the specific flow rate ... 126
Figure 5-25: Monthly values of useful energy (column) and operational hours (line) when varying the specific flow rate ... 127
Figure 5-26: Monthly values of useful energy (column) and operational hours (line) when varying the inlet from the heat source ... 128
Figure 5-27: Monthly values of useful energy (column) and operational hours (line) when varying the inlet from the cold side ... 129
Figure 6-1: Thermal efficiency (line) and useful energy (column) of the water and air based BIPV/T systems ... 133
Figure 6-2: Auxiliary and pump energy consumption and thermal solar fraction of the water based BIPV/T system ... 134
Figure 6-3: Auxiliary and fan energy consumption and thermal solar fraction of the air based BIPV/T system ... 135
Figure 6-4: Monthly operating hours of the water and air based BIPV/T systems ... 135
Figure 6-5: Comparison of electrical efficiency and electrical energy production between technologies ... 137
Figure 6-6: Comparison of PV surface temperatures between technologies in June ... 138
Figure 6-7: Comparison of electrical efficiency between technologies in June ... 138
Figure 6-8 Comparison of electrical efficiency between technologies in November ... 139
Figure 6-9: The electrical production, load covered by production and load for the water based BIPV/T system ... 140
Figure 6-10: The electrical production, load covered by production and load for the BIPV system .. 141
Figure 6-11: The electrical production, load covered by production and load for the air based BIPV/T system ... 142
xvi
List of Tables
Table 1: ZEB renewable energy supply option hierarchy [36] ... 24
Table 2: Mismatch factors and indicators. Adapted from [37]... 26
Table 3: Energy storage systems. Adapted from [5] ... 32
Table 4: Main parameters of the PV/T test rig ... 44
Table 5: Parameters used for simulation of the experimental water based PV/T component ... 50
Table 6: CV(RMSE) and MBE of the two validation metrics for the baseline model. ... 75
Table 7: CV(RMSE) and MBE values for wind induced heat transfer and sky temperature calculations ... 79
Table 8: CV(RMSE) and MBE of the validation metrics for the calibrated and baseline models. ... 82
Table 9: Thermal, electrical and total efficiencies of the baseline model, the calibrated model and the measured data ... 91
Table 10: Building envelope of the GEL office based on values from NS 3701 and TEK 15 minimum requirements [78]. Adapted from [58] ... 98
Table 11: Base case values of the water based BIPV/T system ... 106
Table 12: Optimisation parameters for the water based BIPV/T system ... 107
Table 13: Solar fractions for various system parameters and test ranges ... 118
Table 14: Chosen values of the final water based BIPV/T system ... 119
Table 15: Base case values for the air based BIPV/T system ... 120
Table 16: Optimisation parameters for air based BIPV/T ... 121
Table 17: Solar fractions for various system parameters and test ranges ... 131
Table 18: Chosen values of the final air based BIPV/T ... 132
Table 19: Mismatch factors and thermal solar fraction ... 142
Table 20: Energy needs for various solar technologies ... 143
Table 21: Values for the GEL office construction ... 154
Table 22: Detailed window construction ... 155
Table 23: Internal loads, ventilation rates and system design chosen in accordance with criteria for passive house (NS 3701) ... 156
Table 24: Components of the GEL office in accordance with minimum demands of passive house requirements [78]. ... 156
xvii Nomenclature
A Area [m2] Nu Nusselt number [-]
AC Alternating Current Q Energy [J]
B Anergy [J] Q̇ Rate of heat transfer [W]
C Cloudiness factor [-] q Heat transfer per area [W/m2]
Cp Specific heat capacity [J/kgK] R Heat transfer resistance [m2K/W]
C Speed of light [m/s] R2 Determination coefficient [-]
D Diameter [m] or weighted exported
energy [J] S Absorbed solar radiation [W/m2]
DC Direct Current T Temperature [°C] or [K]
D Delivered energy [J] UL Collector overall heat transfer
coefficient [W/m2] E Energy [J] or weighted exported
energy [J] ŷt Simulated data [-]
e Exported energy [J] yt Measured data [-]
FR Heat removal factor [-] V Opearting voltage [V]
f Solar fraction [-] v Velocity [m/s]
fgrid Grid interaction index [-] W Width [m] or work [J]
fload Load match index [-] X Multiplier for PV cell efficiency [-]
fpar
Fraction of pump power converted to thermal energy [-]
G Irradiance [W/m2] or weighted
energy generation [J] Greek
g Energy generation [J] α Absorptance
h Heat transfer coefficient [W/m2K]
or Planck’s constant [m2kg/s] γ Controller function
I Current [A] or irradiation [Wh/m2] ε Emissivity
IAM Incident Angle Modifier [-] Λ Wave length [nm] or thickness [m]
i Energy carrier η Efficiency
k Thermal conductivity [W/mK] µ Cell temperature coefficient [1/K]
L Length [m] ξ Exergy efficiency
l Energy load [-] σ Stefan-Boltzmann constant
[W/m2K4]
ṁ Mass flow rate [kg/s] τ Transmittance
N/n Number of [-]
P Pressure [bar]
xviii
Subscripts
AC Alternating Current G Energy generation
a Ambient H High
abs Absorber H Hydraulic
back Back surface hor Horizontal
C Cell or condenser in Inlet
c Collector K Time step
cond Conductive L Low
conv Convective L Energy load
DC Direct Current mp Maximum power point
d Diffuse N Normal incidence
dp Dew Point OC Open circuit
e Electrical out Outlet
eq Equivalent P Parallel connection
G Energy generation ph Photon
H High rad Radiative
H Hydraulic ref Reference conditions
hor Horizontal S Series connection or surface
in Inlet T On tilted surface
K Time step th Thermal
L Low tot Total
L Energy load U Useful
mp Maximum power point S Series connection or surface
N Normal incidence T On tilted surface
OC Open circuit th Thermal
out Outlet tot Total
P Parallel connection U Useful
ph Photon
rad Radiative
ref Reference conditions
xix
Abbreviations
AHU Air Handling Unit PV Photovoltaic
AM Air Mass PV/T Photovoltaic Thermal
BAS Building Automation System RMSE Root Mean Square Error
BAPV/T Building Applied Photovoltaic
Thermal SJTU Shanghai Jiao Tong University
BIPV/T Building Integrated Photovoltaic
Thermal STC Standard Test Conditions
CHP Combined Heat and Power STD Standard Deviation
CO2 Carbon dioxide TESS Thermal Energy System Simualtion
Inc.
COP Coefficient of Performance TRNSYS Transient System Simulation Tool CV(RMSE) Coefficient of Variation of RMSE ZEB Zero Energy Building
CZ Czochralski
DHW Domestic Hot Water
FORTRAN Formula Translation GEL Green Energy Laboratory GHG Greenhouse Gas emissions HVAC Heating, Ventilation and Air
Conditioning
IDA ICE IDA Indoor Climate and Energy IDA SE IDA Simulation Environment IWEC International Weather for Energy
Calculations
LPG Liquid Petroleum Gas
MBE Mean Bias Error
NMBE Normalised Mean Bias Error
NMF Neutral Model Format
NOCT Normal Operating Cell Temperature
NREL National Renewable Energy Laboratory
NTNU Norwegian University of Science and Technology
nZEB Net-zero Energy Building
1
1 Introduction
1.1
ObjectiveThe goal of this master thesis is to analyse and develop design methods for building integrated photovoltaic/thermal (BIPV/T) technology at the Green Energy Laboratory (GEL) at
Shanghai Jiao Tong University (SJTU) in China. The work will include analysis of both water and air based PV/T technologies and assess the performance of the water and air based
BIPV/T systems through simulations with the appropriate simulations tools.
A detailed validation and calibration process will be conducted based on experimental data to ensure realistic results from simulations. Computer models will be developed to assess the effects of PV/T technology integrated in the southern façade of an office space in the GEL.
Effects on the building’s energy demand as well as the performance of the solar energy technology will be investigated.
This collaborative assignment is realised as a part of the Joint Research Center in Sustainable Energy of the Norwegian University of Science and Technology and Shanghai Jiao Tong University. The main findings will be incorporated in a scientific paper draft proposal included in the end of the report.
1.2
BackgroundAs the consequence of climate changes are getting increasingly more severe, it is important to reduce the energy production from fossil fuels. The sun represents a huge source of clean, renewable energy which must be utilised to ensure a sustainable way of life and preserve the world as we know it today.
Growing populations and expanding cities present the need for alternative technologies for onsite energy generation. The photovoltaic (PV) cell is an established technology for
production of electrical energy, with efficiency ranging from 5% – 21% depending on the PV material. However, the efficiency of a PV panel decreases for higher PV surface temperatures introducing the need for additional cooling of the panel. With a colder fluid, i.e. air or water, circulating below the PV panel, the PV temperature is kept lower, maintaining higher
efficiency during hours of high solar irradiation.
In the EU, the member states have agreed that all new buildings are going to be nearly zero energy buildings within 2020. Buildings constructed today must follow strict governmental
2
regulations to maintain low energy consumptions and are thus mostly passive houses and net Zero Energy Buildings (nZEB). As the total building energy demand is reduced in modern buildings, the demand for domestic hot water (DHW) becomes relatively larger. Therefore, renewable solutions for covering the DHW demand are becoming more important.
To reduce the amount of imported electricity from the grid, the energy production should follow the load, i.e. high load matching index should be maintained throughout the year. As solar energy is impossible to regulate, energy storage could be used to minimise grid stress.
BIPV/T is space efficient as it utilises less area for electrical and thermal energy generation compared to traditional solar collectors and PV panels. Façade integrated PV/T systems can make use of an area that has, until now, been found unfitted for energy production. In contrast to PV panels and solar collectors installed on buildings, BIPV/T components offer
architectural uniformity as all components are identical.
1.3
LimitationsMeasurements of the water based PV/T component were conducted in December, and no measurements from summer operation were available to the authors. Ideally, the water based PV/T model should be calibrated and validated for summer measurements in addition to winter measurements, to ensure that the results of simulations are as realistic as possible for the entire year.
No measurement data was available for the air based PV/T component. Thus, the TRNSYS model for this component is not validated, as the air based was, and the development of the air based BIPV/T model is solely based on simulations.
It was not possible to test the entire desired range of flow rates in TRNSYS for the water and air based BIPV/T system models. The highest tested flow rate was 24 kg/hm2 and 63.56 kg/hm2 for the water and air based systems, respectively.
As the air and water based BIPV/T technologies operate with different fluids, it is difficult to find a neutral common ground for comparison. The system in this thesis investigates the production of thermal energy for domestic hot water, which introduces the need of a heat exchanger between the BIPV/T component and the tank in the air based BIPV/T. The heat exchanger efficiency limits the amount of heat transferred from the BIPV/T component to the tank, compared to the water based system where the water heated in the BIPV/T component is transferred directly.
3
1.4
OutlineChapter 2 presents the relevant theoretical background of the thesis. PV and solar collector technologies are explained in detail, as well as PV/T technology as a combination of the two.
Air and water based PV/T are introduced, with a focus on building integration of each one.
The concept of nZEBs are explained, with regards to building design, load matching and smart grids. Lastly, relevant considerations for BIPV/T system design are addressed, as well as effects of coupling a BIPV/T component to a heat pump system.
The GEL at SJTU is presented in chapter 3. The apartment on the 2nd floor is modelled as an open office space to simulate the DHW and electrical demand of an office. This model is used for analysing the air and water based BIPV/T in chapter 5. The measurement data received from fellow GEL students and the parameters of the test component is also presented in this chapter.
The modelling process is explained in detail in chapter 4. The mathematical model of the air and water based TRNSYS PV/T models, TYPE 568 and TYPE 563 are presented. In this chapter, a validated and calibrated water based PV/T model is developed for use in further simulations and analysis conducted in chapter 5. The water based PV/T model was validated according to the measurements presented in chapter 3. In the end, the results of the calibrated model are presented.
Chapter 5 includes the modelling approach of the GEL office model, which was described in chapter 3. The office model is connected to an air based and a water based PV/T component presented in the previous chapter. The result is two GEL BIPV/T system models which are used for simulation to analyse the effect of building integration of air and water based PV/T.
Chapter 6 presents the simulations results from the GEL BIPV/T system models developed in chapter 5. The air and water based BIPV/T systems are compared to air and water based PV/T systems as well as a BIPV system, separately connected to the GEL office.
In the conclusion of the thesis in chapter 7, the objectives of the assignment are answered based on the results found in chapter 6.
Chapter 8 presents suggestions for further work. These are possible extensions of the work conducted in this thesis.
4
1.5
Research MethodsA literature review was conducted to present an overview of the PV/T and BIPV/T
technology. This makes up the basis for the theoretical background presented in chapter 2.
Also, literature review was used to evaluate various simulation tools and determine which one was most suitable for this assignment.
Data for measurements were used as a basis for validation and calibration of the water based PV/T model. The measurements were not conducted by the authors, but by fellow GEL students. Uncertainties of the measurement components (thermometer, flow meter etc.) have not been addressed as these data were not available.
TRNSYS simulations have been conducted, both for validation and calibration purposes, but also for assessing the long-term performance of the BIPV/T systems.
5
2 Theoretical Background
The sun is by far the largest source of energy known to man. Within one hour, an amount equal to the entire energy need of the human population is supplied from the sun. Utilising this energy efficiently is more important now than ever as the world needs a shift from fossil fuels to renewable energy sources.
The mean annual global irradiance incident on a surface horizontal to the ground is shown in Figure 2-1. The highest values are present in the Tropics of Cancer and Capricorn. The irradiance at the Equator is slightly lower than that of the Tropic of Cancer due to an increase of cloud cover. Mean annual global irradiance includes both direct normal irradiance and diffuse horizontal irradiance and is determined by the latitude, current season, time of day, inclination of the surface, shading, orientation and the climatic conditions.
Figure 2-1: Mean annual global irradiance incident on a horizontal surface in W/m2 [1]
2.1
PhotovoltaicPhotovoltaic (PV) technology uses solar cells to generate electricity from solar radiation [1].
Initially, the PV technology was developed for the space industry where space projects were not limited by the cost of materials. However, as production costs of PV have dropped for the last decades, PV is applicable for residential and commercial use.
6
Photovoltaic technologies can be grouped into three categories: First generation, second generation and third generation. The first and second generations are the most commonly used and will be described in further detail in this thesis.
First generation technology photovoltaic cells include both mono- and multicrystalline silicon cells. The second generation is often referred to as thin films, and includes amorphous silicon, cadmium telluride and copper indium gallium diselenide materials cells. [2]
Recently, there has been a vast development within what is referred to as the third generation of solar cells. It includes organic and polymer based solar cells, introducing new organic- inorganic hybrid materials such as perovskite. They provide easier scalability and aims to reach higher efficiencies at reduced costs compared to the two previous generations. The new generation shows great potential, but is still in the developing stages with few commercially available products and is therefore not given any further consideration in this thesis. [3, 4]
2.1.1 Manufacturing
The PV industry relies mainly on discarded second grade silicon from the semiconductor industry. The solar grade silicon consists of up to ten times as much impurities than that of the semiconductor grade silicon, but still it provides sufficient efficiencies. [5]
The Czochralski (CZ) method is the most commonly used technique to transform silicon discards in to crystallised silicon wafer. In the manufacturing process, the silicon is added a small amount of boron to create a p-type base. Furthermore, the wafer is added an n-type semiconductor to create the p-n junction in-between, as well as additional metal layers to conduct electricity. An illustration of a PV cell is shown in Figure 2-2. [5]
Monocrystalline silicon cell manufacturing is a highly energy consuming process. Thus, the manufacturing of multicrystalline and thin film silicon cells are becoming increasingly more common, as they require less energy for production. [5]
7
Figure 2-2: Photovoltaic cell [6]
2.1.2 Photovoltaic Effect
When sunlight hits the photovoltaic material, the energy of the photon is absorbed by an electron in the valence band. If the photon energy is larger than the energy of the bandgap, it will cause the electron to be excited from the valence band to the conduction band, where it is now free to move. However, if the energy of the photon is smaller than that of the bandgap, it will be lost as heat. The energy transported by photons are given in eq.(1) where 𝜆 is the wavelength, h is Planck constant and c is the speed of light. [6]
ph
E hc
(1)
The p-n junction formed in the boundary layer between the two semiconductors creates an electrical field assisting the flow of electrons through the solar cell. The current through the junction depends on an external load being applied to the circuit and the presence of sunlight incident on the photovoltaic material, as illustrated in Figure 2-2. [1]
8 2.1.3 Monocrystalline
Monocrystalline is the purest grade of silicon, and thus has the highest efficiency of the commercially available products. However, it also requires the largest amount of energy for production, about 100 kWh/kg with the CZ technique. The highest quality of monocrystalline silicon is achieved by using a process called Float Zone. [5, 7]
2.1.4 Multicrystalline
The production costs for monocrystalline silicon cells are large, mainly because of the energy intensive production process. In order to reduce the cost, several crystallisation techniques have been developed, such as the solidification method. The result is a less energy intensive process, but the quality of the crystallised silicon is reduced due to imperfections and impurities in the material. Directional solidification uses about 10 – 15 kWh/kg in the production of multicrystalline silicon. [5]
2.1.5 Thin Film
The thin film technology is promising due to reduced material and energy needs in the production phase. Thin film materials can absorb just as much photon energy as that of crystalline silicon, but with a thinner structure. This makes it more applicable for non-flat surfaces, such as curved façades, car roofs or even integrated in clothing for charging of small devices. [5, 8]
For now, commercial thin film materials are outperformed by thicker and more robust panels, e.g. crystalline silicon, when it comes to efficiency and life time of the material. The four most common thin film materials are amorphous silicon, copper indium diselenide, and cadmium telluride.
In 2015, the production of thin film solar cells was 7 % of the annual production of solar cells, which is dominated by multicrystalline silicon cells, as shown in Figure 2-3. [5, 7]
9
Figure 2-3: Market shares of photovoltaic technologies. Data from [7]
2.1.6 Performance of Photovoltaics
The highest PV efficiencies are reached during laboratory measurements. These efficiencies are usually not valid for the actual commercialised PV panels, which usually report somewhat lower efficiencies. National Renewable Energy Laboratory (NREL) mapped the highest recorded efficiencies, measured under standard test conditions (STC1), of various PV technologies from 1976 to 2016. The results are shown in Figure 2-4:
1 STC is an acronym for Standard Test Conditions for solar cells: Irradiance of 1 000 W/m2, air mass 1.5 (AM1.5) spectrum and a temperature of 25°C. [5]
10
Figure 2-4: Highest measured PC cell efficiencies [9]. This plot is courtesy of the National Renewable Energy Laboratory, Golden, CO
11 The Shockley-Queisser Efficiency Limit
The detailed balance limit of efficiency for PV cells, known as the Shockley-Queisser efficiency limit, is the upper theoretical limit of an ideal p-n junction solar photovoltaic cell [10]. The value was found to be 33.77% for a band gap energy of 1.34 eV. The limit provides a benchmark for the maximum performance of a single junction PV cell. As silicon has a band gap energy of 1.1 eV, the highest achievable theoretical efficiency is, according to the
Shockley-Queisser efficiency limit, 32.23%. [11]
Temperature Effect
The solar cell power output is dependent on the temperature of the cell. An increase in cell temperature will reduce the cell voltage with about 2.3 mV per °C for a silicon cell. The relationship between the ambient temperature, Ta, and the operating cell temperature, TC, is dependent on the normal operating cell temperature, NOCT2, and given as: [5]
20
C 0.8 a
T NOCT G T (2)
where G is the solar irradiance.
Commercialised PV Efficiency
In laboratories, the aim is to achieve as high efficiency as possible, and the cell is developed based on that one goal in particular. Little concern is given to the economical aspect including the lifetime of the cell and application. For the commercial PV panels, the economical aspect is more important compared to the application in laboratories. Whether a PV panel is
economically sound or not may be the deciding factor for commercial manufacturing, and is therefore prioritised at the sacrifice of highest possible efficiency.
The average efficiency of a commercialised monocrystalline panel is 17%, with a maximum of 21%. For the multicrystalline panels, common efficiencies range from 13 to 17%. The efficiencies for the commercial thin film solar cells range from 5 to 11%, with amorphous silicon providing the best efficiency. [5]
2 NOCT is an acronym for the Normal Operating Cell Temperature for a solar cell. It is given under the following conditions: Irradiance of 800 W/m2, AM1.5 spectrum, ambient temperature of 25°C and wind speed
>1 m/s. [5]
12
The maximum power point efficiency of a PV cell is defined as the maximum power
produced by the cell under STC, divided by the solar radiation incident on the cell, shown in eq.(3):
mp mp mp
c T
I V
A G (3)
where 𝐼𝑚𝑝 and 𝑉𝑚𝑝 are the maximum power point current and voltage, respectively. The solar irradiance incident on the tilted collector area, Ac, is denoted GT. As the cell efficiency drops at higher cell temperatures and vice versa, Duffie and Beckmann [12] described the
temperature dependence of the cell efficiency through eq.(4):
, , ,
mp mp ref mp TC TC ref
(4)
where 𝜂𝑚𝑝,𝑟𝑒𝑓 is the efficiency measured at STC, 𝜇𝜂,𝑚𝑝 is the cell efficiency temperature coefficient, 𝜂𝑚𝑝 is the maximum cell efficiency and 𝑇𝐶 and 𝑇𝐶,𝑟𝑒𝑓 are the cell temperature and the cell temperature at STC, respectively.
The cell efficiency temperature coefficient is obtained by solving the equation for the
maximum power efficiency over a range of temperatures, as shown in the following equation:
,
1
mp mp mp
mp mp mp
c T
d dV dI
I V
dT dT dT A G
(5)
As 𝜇𝜂,𝑚𝑝is small for many PV panels, the value of d𝐼𝑚𝑝
d𝑇 is regarded as equal to zero and d𝑉𝑚𝑝
d𝑇
considered equal to d𝑉𝑂𝐶
d𝑇 . As a result of these approximations, the temperature coefficient of the maximum power efficiency can be given as:
, ,
mp OC OC
n mp mp ref
c T mp
I dV V
A G dT V
(6)
where VOC is the open circuit voltage, i.e. the voltage at I = 0 A.
2.2
Solar Thermal CollectorsThe basic principle of solar thermal collectors involves absorption of solar radiation and heat exchange by running a working fluid through a heat exchanger. The thermal energy output is then often used for domestic hot water, hydronic heating or industry related heating. [13]
13
There is a large variety of solar thermal collectors available, and they are primarily grouped into stationary collectors and collectors that change position by single or dual axis solar tracking. In addition, a distinction is made between collectors that use different heat transfer medium (e.g. water and glycol mixture, air or heat transfer oil), concentrated or non-
concentrated collectors, as well as covered or uncovered collectors. [6]
In this thesis, an uncovered, flat plate PV/T component is analysed and thus only the flat plate solar collector will be described in further detail. The other main types of commercial solar collectors are concentrating solar collectors and evacuated tube collectors. Also, it should be mentioned that there is vast ongoing research in the field of solar collectors, but these technologies are not yet commercialised.
2.2.1 Flat Plate Solar Collector
A flat plate collector absorbs shortwave solar radiation, which is then used to heat the working fluid flowing through the pipes in the collector. The insulated frame covers the area of the pipe that is not in contact with the absorber, and thus reduces the conductive heat loss from the pipes. In addition, a transparent cover may be added to allow short wave radiation to pass and reflect longwave radiation emitted by the absorber, consequently reducing the radiative heat loss from the collector (greenhouse effect). [13]
There is also a convective heat loss from the collector, which is dependent on local conditions such as wind speeds and ambient temperatures. The convective heat loss is reduced by using a transparent cover as shown in Figure 2-5, and even more with a double-glazed cover.
However, this will also reduce the overall transmittance-absorptance product of the collector.
[13, 14]
Figure 2-5: Covered flat plate collector [13]
The solar collectors should face towards the equator in order to optimally harvest solar energy. The optimal tilt angle for a flat plate collector is equal to the latitude of the site plus
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10° if the system is designed for optimal winter performance or minus 10° for optimal summer performance [6].
Covered flat plate collectors are most common for building integration as they can easily replace conventional building materials and serve as a wind barrier in the construction [15].
Flat plate collectors represent 22% of the global installed solar collector capacity, which is dominated by evacuated tube collectors (numbers from 2014), as show in Figure 2-6:
Figure 2-6: Market shares of installed solar collector capacity. Data from [16]
2.2.2 Performance of Solar Collectors
The useful energy gain from a solar collector, Q̇u, is represented by an energy balance including the incoming solar radiation on an absorber area and the thermal and optical losses from the collector. The thermal and optical losses are combined and shown as UL multiplied by the difference between the mean absorber plate temperature and the ambient air
temperature: [12]
[ ( )]
u c L abs a
Q A S U T T
(7)where:
S = GT(τα), absorbed solar radiation, gives Qu [W], as GT [W/m2] is irradianceand (τα) is the transmittance-absorptance product.
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S = IT(τα)avg. Meteorological data are mostly given in time steps of one hour,
therefore it is often preferred to integrate GT over an hour, into irradiation, IT. The transmittance-absorptance product, (τα)avg, is the average of the same period. [12]
UL = Collector heat and optical losses [W/m2K]
𝑇̅𝑎𝑏𝑠 = Mean absorber plate temperature [K]
Ta = Ambient air temperature [K]
The mean absorber plate temperature, Tabs, is difficult to quantify as it depends greatly on the overall design and performance of the collector. Therefore, an adaptation of eq.(7) which replaces the mean temperature of the absorber plate with the fluid inlet temperature is often preferred. This is done by introducing a heat removal factor, FR, as shown in the following equation:
[ ( , )]
u c R L fluid in a
Q A F S U T T (8)
where Tfluid,in is the fluid inlet temperature and FR represents the ratio between the actual collector output and the hypothetical output if the mean absorber plate temperature was the same as the fluid inlet temperature. FR is given as:
, ,
,
( )
[ ( )]
p fluid out fluid in R
c L fluid in a
mC T T
F A S U T T
(9)
where:
ṁ = Mass flow rate in the collector [kg/s]
Cp = Specific heat capacity [J/kgK]
Tfluid, out = Fluid outlet temperature [K]
The solar collector efficiency measured over a period of time is given as:
u th
c T
Q dt A G dt
, (10)16
2.3
Building Integrated PV/T ComponentAs described in subchapter 2.1.6, the PV panel efficiencies are limited to the range from 5%
to 21%, the remaining solar irradiance which is not reflected is converted to heat. This heat will increase the cell temperature in the PV panels, which will lead to a reduction in power output. This has led to the desire to cool down the PV panels, utilising the thermal output from the PV for heating purposes in the process. This process is known as photovoltaic thermal (PV/T) technology. PV/T components that are architecturally and functionally integrated into the building envelope are called building integrated photovoltaic thermal (BIPV/T) components. The BIPV/T differs from the building applied PV/T (BAPV/T) as the BIPV/T components replace structural materials such as roof shingles and wall cladding. The BIPV/T component may serve several purposes as it can be used as a barrier against the weather and noise, in addition to generating electrical and thermal energy. [13]
In urban areas, people live denser than in rural areas and high-rise buildings are common in many cities. This means that the rooftop area is very small compared to the overall energy use of the building. As the BIPV/T technology can be integrated into the façade as well as the roof, a bigger area can be utilised for electricity and heat production. Also, the combination of the two technologies may show that there is no need to compete for the same roof or façade area, as both electrical and thermal output is provided.
There is a vast amount of technologies of BIPV/T as seen in Figure 2-7. In addition, there are combinations which include the different photovoltaic materials described in subchapters 2.1.3 – 2.1.5. The types of BIPV/T systems that will be evaluated in this thesis are the water and air based uncovered flat plate BIPV/T.
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Figure 2-7: Overview of various BIPV/T technologies [17]
2.3.1 Air Based BIPV/T
In the air based BIPV/T component, heat is transferred from the PV panel to cooler air flowing through an air channel beneath the PV panel. A principle sketch of a BIPV/T air component is shown in Figure 2-8:
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Figure 2-8: Air based BIPV/T component
The air flow is driven by either thermosyphon effects for free circulation or a fan for forced circulation. The heated air from the BIPV/T may be used in an open loop or closed loop system. In the open loop system, the heated air is used directly in the building ventilation system. It is mixed with the ambient air to regulate the temperature of the air entering the building and thus reduces the building energy consumption. In the closed loop system, the air is used indirectly in a heat exchanger connected to e.g. a DHW tank to be used in a high temperature system. Figure 2-9 (a) and (b) show principle system sketches of open and closed loop systems, respectively: [13]
Figure 2-9: Open loop (a) and closed loop (b) air based BIPV/T systems. Adapted from [13]
Generally, the electrical efficiency is higher in open loop systems than in closed loop systems.
Although some of the heat is extracted in the heat exchanger in the closed loop system, the returning air entering the BIPV/T is not as cool as for the open loop system and therefore the PV temperature gets higher in the closed loop system. [13]
19
The energy balance of the air based BIPV/T is described in detail in subchapter 4.1.2.
2.3.2 Water Based BIPV/T
The water based BIPV/T component is similar to the air based, but instead of air flowing in an air channel, water circulates in tubes beneath the PV panel, as seen in Figure 2-10:
Figure 2-10: Water based BIPV/T component. Flow direction into the page
The principle is the same as for the flat plate solar collector described in subchapter 2.2.1. As for the air based BIPV/T, the fluid flow is free or forced, i.e. driven by thermosyphon effects or by a pump. The water based BIPV/T is used in closed loop systems and integrated into the building envelope so that the back of the component acts a part of e.g. the building wall construction.
The energy balance of the water based BIPV/T is described in detail in subchapter 4.1.1.
2.3.3 Performance of BIPV/T
Typically, air based BIPV/T system have reduced thermal performance due to lower density, thermal conductivity and specific heat capacity of air than water. In the case of thermosyphon fluid circulation, the power consumption of the pump or fan is eliminated. However, the design of the BIPV/T component is of high importance to ensure sufficiently large mass flow rate in the system. Therefore, façade integration may be more beneficial compared to roof integration to make best possible use of the buoyancy effect.
The air based BIPV/T system requires less maintenance than the water based system, and potential leakages will not cause significant damage to the system. However, for the closed loop BIPV/T air system, the increased PV cell temperature will accelerate component deterioration. [13]
20 BIPV/T Efficiencies
The total efficiency, ηtot, is the sum of the thermal and electrical efficiencies:
tot th e
(11)Previous studies documenting the total efficiency of uncovered BIPV/T water or air components have proven difficult to find. Studies by Kim et al. [18] and Athienitis [19]
reported total efficiencies of 47% and 55%, respectively, for corresponding uncovered BIPV/T water and air components. Studies on uncovered PV/T [20-24] report thermal
efficiencies ranging between 45% – 60% for water based and 38% – 46% for air based PV/T.
Electrical efficiency from 9.5% to 14.5% was reported for water based PV/T and from 10.4%
to 13% for air based PV/T.
Studies on other BIPV/T [25-32] report thermal efficiencies between 37.5% – 72% for water based BIPV/T and between 17.2% – 53.7% for air based BIPV/T. Electrical efficiencies in the range 4.9% – 11.6% and 10% – 15.5% were reported for water and air based BIPV/T,
respectively. These numbers provide a rough basis of comparison for the magnitude of the expected thermal and electrical performance of the uncovered BIPV/T component analysed in this thesis. However, measurements and simulations should be carried out to determine the performance of the specific component.
Exergy and Anergy
The total efficiency, ηtot, accounts for both thermal and electrical energy, but not the quality of the energy. The exergy, often referred to as available energy, is the amount of energy that can be transformed into other forms of energy without losses, and thus addresses the energy quality. The counterpart of exergy is anergy (unavailable energy). Electrical energy consists solely of exergy, whereas thermal energy is part exergy and part anergy. The correlation between energy, exergy and anergy is given in Eq.(12):
Energy Q Exergy E Anergy B Constant (12) The exergetic part of the thermal energy, the thermal exergy, is limited by the Carnot
efficiency, ηCarnot: [33]
sink source 1 source
Carnot
sink sink
T T T
T T
(13)
